NORTHEASTERN FOREST EXPERIMENT STATION 

Upper Darby , Pennsylvania 

No* 40 

October 1954 

Swnmer Is The ,Best Tjme 

To. Thin Hybrid Poplar Plantations 

Hybrid poplar plantations are established by planting 

a dormant cuttings in close spacing, usually 4 x 4 feet. They 

are cultivated during the first growing season to elbinate 

competition from grasses and weeds, After the first year, 

the more vigorous trees effectively shade out lower vegetation. But rapid tree growth often makes thinning necessary 

after 2 or 3 growing seasons, 

An ax or machete is the common tool used in thinning 

hybrid poplar plantations. The trees are usually less than 

2 inches in diameter, so they can be easily cut near the 

ground line. Often the stumps of cut trees sprout, and may 

again offer competition after several years of growth. 

Two hybrid poplar plantations on the Beltsville Experimental Forest, near Bowie, Maryland, were used in a 

study of thinning. One was thirined during the summer growing season, the other during the following winter 'dormant 

season. After 2 years, detailed records of sprouts from all 

cut stumps were obtained. There were about 18 cut stumps in 

each of 11 plots on each area. The same hybrid poplar clones 

were present in both plantations. 

The results of the sprout measurements are summarized 

in table 1, Data for individual clones are not given since 

no statistically significant differences between clones were 

found. 

The number of sprouts per stump varied from 1 to 30. 

Although the average number of sprouts from the stumps of 

winter-cut trees was greater than from those cut in the 

summer, this difference was not statistically significaht. 

But diameter differences were important. Sprouts 

from the stumps of trees cut in the summer had nearly a half 

season more to grow than sprouts from the stumps of trees 
? 

Table 1. --Summaw of sprouting by 11 hybrid poplar*clones , . 

24 growing seasons after thinning 

Sprouts Average The Of Average , Stumps per stm~, diameter weeding height ' with sprouts average (b.h.) 

I 

Number Inches Feet Percent 

Summer 4.6 0.4 6.1 57.9 

Winter 7.1 .7* 11.8* 96. S* 

I 

*~ifferences between summer and winter thinning statistically significant. 

cut in winter. Even with this advantage, their average diameter was about half that of sprouts from winter-cut stumps. 

This slower growth may have been due in part to depletion of 

food by the new summer sprouts, when food storage normally 

takes place. As a result the top and roots had fewer food 

reserves next spring when growth started again. 

Sprouts from winter-cut stumps were almost twice as 

tall as those from stumps cut in the summer. The smaller 

sprouts on the summer-cut stumps offer considerably less 

competition to the residual trees. Since the crop trees are 

growing rapidly, these smaller sprouts will probably never 

be serious competitors. 

Another significant advantage of sumtner thinning is 

the smaller number of stumps with living sprouts 2 years 

later. At that time nearly all the winter-cut stumps had 

living sprouts, while only about 58 percent of the summercut stumps had sprouts. Again, depletion of food reserves 

may have been the cause. 

The results of this study indicate that summer thinning is better than winter thinning for two reasons. First, 

the stumps have fewer sprouts. Second, the surviving sprouts 

are about half the size of those fromthe stumps of trees 

cut in the winter. Thus, thinning during the summer more 

effectively achieves the purposes of thinning in hybrid 

poplar plantations. 

--HAROLD F. FORD and AUERT G. SNOW, JR. 

Virginia Pine-Hardwoods Research Center 

Northeastern Forest Experiment Station 

Forest Service, U.S. Dept. Agriculture 

Agriculture-Forest Service-Upper Darby, Pa.